1. Field of the Invention
The present invention relates to a method of manufacturing an exhaust gas purifying catalyst in which alumina layers supporting therein catalytic metals are formed on the surface of a metallic carrier.
2. Description of the Prior Art
Conventionally, exhaust gas purifying catalysts in which catalytic metals are supported in a honeycomb-like ceramic carrier and those in which catalytic metals are supported in gamma-alumina layers formed on the surface of a metallic carrier are commercially available as exhaust gas purifying catalysts for an automotive internal combustion engine.
Since the latter are superior in opening ratio, geometrical surface area and thermal capacity to the former, the scope of application thereof is gradually increasing.
In the meantime, in the exhaust gas purifying catalysts employing a metallic carrier, when the bonding force between a metallic carrier of stainless steel or the like and alumina layers is not sufficient, the alumina layers supporting catalytic metals tend to be separated during the use for a relatively short period, thus lowering the exhaust gas purifying efficiency. Accordingly, it is necessary to tightly bond the metallic carrier and the alumina layers to each other.
U.S. Pat. No. 4,279,782 discloses a method of coating an oxide whisker-covered surface. In this method, a metallic carrier made of heat-resistant alloy steel containing aluminum is initially subjected to a special heat-treatment so that needle-like whiskers of alumina may be formed on the surface of the metallic carrier. Aqueous alumina gel containing no gamma-alumina particles is then coated on the surface of the metallic carrier, and in a wet state, aqueous alumina gel containing gamma-alumina particles is coated thereon. Thereafter, these gels are dried and calcined.
When the aqueous alumina gel containing no gamma-alumina particles permeates between the whiskers and when the aqueous alumina gel containing gamma-alumina particles is coated thereon, a series of alumina gel matrixes are formed around the gamma-alumina particles and the whiskers. Upon drying and calcining, alumina in the alumina gel matrixes turns into gamma-alumina, which tightly bonds the gamma-alumina particles and the whiskers to each other, thereby strengthening the bonding force between the metallic carrier and the gamma-alumina layers formed on the surface thereof.
Japanese Patent Application (examined) No. 58-23138 discloses a catalyst provided with a metallic carrier, in the surface of which catalytic metals are supported. In this catalyst, oxidation treatment is applied to the surface of a metallic carrier of heat-resistant alloy steel containing aluminum to form an oxide aluminum coating, and alumina layers are then formed thereon.
However, in the method as disclosed in U.S. Pat. No. 4,279,782, it is necessary to make use of a metallic carrier manufactured by a special manufacturing method or to apply a specific heat-treatment to the metallic carrier in order to form alumina whiskers on the surface of the metallic carrier. As a result, the manufacturing cost increases. Furthermore, the metallic carrier and the alumina layers are bonded via a number of alumina whiskers, and when the alumina whiskers are formed on the surface of the metallic carrier, an aluminum oxide coating is also formed thereon. Because of this, there arises the problem of the presence of a limit in enhancing the bonding force between the metallic carrier and the alumina layers.
In the catalyst as disclosed in Japanese Patent Application No. 58-23138, an oxidation treatment process is required to form an aluminum oxide coating on the surface of the metallic carrier. As a result, the number of process increases and the manufacturing cost becomes expensive. Furthermore, after the formation of hydrated alumina on the aluminum oxide coating formed on the surface of the metallic carrier, the hydrated alumina must be dried and calcined to form a gamma-alumina layer. Because of this, there arises the problem of the presence of a limit in enhancing the bonding force between the aluminum oxide coating and the gamma-alumina layer.